The present invention relates to a wheel alignment technique. In particular, the present invention relates to measurement of camber and pitch in a wheel alignment system.
Proper alignment of the wheels of an automotive vehicle is important for proper handling of the vehicle and proper tire wear. In addition, proper alignment of the wheels will decrease fuel consumption as well as increase vehicle safety. The wheel alignment parameters which are measured and adjusted in order to achieve proper wheel alignment are camber, caster, steering axis inclination (SAI) and toe.
Camber angle is the inclination of the wheel plane with respect to vertical. It is considered positive when the wheel leans outward at the top, and is considered negative when it leans inward. Any wheel of an automobile can have camber.
Caster angle is the angle in side elevation between the steering axis of a steerable wheel (typically one of the front wheels) with respect to vertical. It is considered positive when the steering axis is inclined rearward (in the upward direction) and negative when the steering axis is inclined forward.
Steering axis inclination (SAI) or king pin inclination is the angle in the front elevation between the steering axis and vertical.
The static toe angle of a wheel, at a specified wheel load or relative position of the wheel center with respect to the sprung mass, is the angle between a longitudinal axis of the vehicle and the line of intersection of the wheel plane and the road surface. The wheel is "toed-in" if the forward portion of the wheel is turned toward a central longitudinal axis of the vehicle, and "toed-out" if turned away.
Camber, caster and SAI are typically measured using inclinometers attached to the wheel. With camber, the inclinometer measures the angle that the plane of the wheel makes with the vertical. To measure caster, the wheel is turned through an arc, and the difference in the camber readings is used to derive the caster value. SAI is measured in a manner similar to caster, except that the inclinometer used for the SAI reading measures the change in pitch angle of a line in the plane of the wheel as the wheel is turned through an arc. The SAI measuring inclinometer is aligned at 90.degree. to the inclinometer used for reading camber and caster.
Alignment systems can also use the pitch angle (inclination angle) of head units in calculation of alignment angles. This is described in U.S. Pat. No. 5,014,227 entitled CASTER AND STEERING AXIS INCLINATION MEASUREMENT TECHNIQUE, which is incorporated herein by reference.
In prior art alignment systems, camber and pitch sensors must be individually aligned for each head unit. Improper sensor alignment is an additional error source. Errors in measured caster and measured pitch can prevent proper wheel alignment. A wheel alignment system which improves accuracy in wheel alignment measurements would be an important contribution to the art.